(NYTIMES) - More than 448 million doses of coronavirus vaccines have been administered worldwide, and in some countries immunisation campaigns are allowing people to resume quasi-normal life.

In Israel, where 50 per cent of the population has been immunised, residents can show a "green pass" - proof that they have been vaccinated - to enter restaurants, theatres and gyms.

Both the European Union and China recently announced their own versions of vaccination passes. While vaccination certificates may allow holders to enter businesses within a country, governments are also hoping to use them to regulate international travel and borders.

But in much of the world, coronavirus vaccines remain in short supply and, in some cases, wholly unavailable.

As governments, largely in rich countries, seek to use vaccine passports to relax restrictions, they risk relying on a fragmented system that could have the adverse effect of extending the pandemic.

In addition to the vaccine passes being prepped in Europe and China, the World Economic Forum is working with a group called the Commons Project on a system for documenting coronavirus vaccinations.

IBM is developing a Digital Health Pass, and the International Air Transport Association, a trade association for the airline industry, is developing a smartphone app that will provide passengers with information about testing and vaccination requirements.

Mandating vaccine passports for international travel introduces several challenges.

International law allows countries to require visitors to prove they have been vaccinated against diseases like yellow fever. But coronavirus vaccines are new, and they have not all been authorised for use throughout the world.

Questions of which vaccines and new variants

Countries may decide to accept evidence only of vaccines approved for use within their borders. China, which is looking at implementing differentiated policies for visa issuance, has linked plans to ease border restrictions for foreigners with their getting China-made Covid-19 vaccines.

None of the vaccines currently available in the United States are made by a Chinese company. The Oxford-AstraZeneca vaccine is being used in 86 countries, but it is not approved for use in the US. And vaccines administered in some nations may not be effective against new variants that emerge either domestically or abroad.

These issues need to be addressed on a global scale, and governments should take the opportunity to tackle these questions at the meeting of the World Health Organisation's World Health Assembly in May.

Vaccine passport systems should clarify which shots will be accepted, and they should be equipped to update immunisation requirements when public health guidance changes.

These systems must also prevent countries from arbitrarily refusing to accept certificates. Without international consensus, we risk entrenching geopolitical divides with inconsistent requirements that could prolong the pandemic.

Vaccine passports that enable citizens of some nations to travel internationally while millions of others wait for vaccinations will serve only to deepen global inequities.

The United States has not announced plans for a vaccine passport. Americans who get vaccinated receive a card indicating the date and type of shot they received, but those cards are not being used as passes and can be easily forged. And an American plan would present a number of issues.

Ethical, legal and logistical issues

First, the vaccine roll-out in the US has been plagued by inequities: black and Latino people are being vaccinated at much lower rates than white people. Second, not everyone can get vaccinated: The shots have not been approved for children, and the data on the safety of vaccinations during pregnancy is limited.

Vaccination passes will need a mechanism to ensure that people who cannot get the shots are not denied jobs, services or education.

A requirement by employers and businesses of proof of vaccination would carry not only ethical and legal hurdles, but also logistical challenges surrounding how that data is collected, stored, verified and protected.

Businesses that require vaccinations for customers or employees will need systems for reviewing vaccine passports, which could create a significant financial burden for struggling businesses.

While schools and healthcare providers have long required and tracked certain immunisations, many companies have never needed to contend with vaccine requirements.

If vaccine passports come in the form of a smartphone app, some people would not be able to use them. And, of course, since a vaccine passport records private health data, a failure to protect this information would create large risk of fraud, counterfeiting, discrimination and privacy violations.

People around the world are eager for the pandemic to end, and those who are vaccinated are understandably eager to take advantage of the freedom that immunisation promises. But any moves to institute vaccine passports must be coordinated internationally and should be coupled with global and equitable access to vaccines.

• Saskia Popescu is an infectious disease epidemiologist at George Mason University who has advised the World Health Organisation (WHO) on infection prevention. Alexandra Phelan is a global health lawyer at Georgetown University who has advised the WHO on legal issues related to infectious disease.